At present, the conventional cyanide leach process is used almost universally to extract gold. However, the growing concern over safety and environmental hazards associated with the use of cyanide combined with the limited use of the process for treatment of problematic gold ores (e.g., refractory sulfide ores, carbonaceous ores, ores containing significant amounts of cyanocides) requires development of an economically viable, alternative leach process.
Gold is readily dissolved in acidified solutions containing thiourea, using manganese dioxide, hydrogen peroxide or ferric ions as the oxidant D. W. Bilston, R. N. Millet and J. T. Woodcock (1987) Proc. Confer. "Research and Development in Extractive Metallurgy" (Aus. I.M.M. Melbourne) pp. 19-28]. However, this gold dissolution process has found little application in the gold industry. The main drawback has been the excessive oxidative degradation of thiourea which occurs in the presence of the above mentioned oxidants. The relevant chemical reactions can be written in two stages involving initial oxidation to formamidine disulfide(1), followed by further irreversible decomposition to sulfur, cyanamide and thiourea(2). EQU Fe.sup.3+ +tu.fwdarw.1/2(tu).sub.2.sup.2+ +Fe.sup.2+ ( 1) EQU (tu).sub.2.sup.2+ .fwdarw.sulfinic compound.fwdarw.S+CN(NH.sub.2)+tu(2)
where tu=thiourea
and (tu).sub.2.sup.2+ =formamidine disulfide.
Reaction (1) is much faster than reaction (2) and was considered as the dominant reaction in determining the relative consumption of thiourea. Furthermore, the rapid consumption of thiourea during the leach process is often associated with passivation of gold in the feed material, and hence low gold recoveries. Consequently, in the past, the thiourea leaching process has proved very unattractive in comparison with the universally accepted cyanidation process.
In recent work it has been shown that the use of sulphur dioxide in conjunction with thiourea significantly reduces reagent loss through oxidative degradation (R. G. Schulze (1984) J. Metals 36 (6) pp. 62-65). However, the lowering of thiourea consumption by using sulphur dioxide has not always been achieved in practice D. W. Bilston, R. N. Millet and J. T. Woodcock (1987) Proc. Conf. "Research and Development in Extractive Metallurgy" (Aus. I.M.M. Melbourne) pp. 19-28. The same workers have also patented the use of a substituted thiourea, i.e. ethylene thiourea. This particular derivative is more stable to oxidation and exhibits a far greater resistance to oxidative degradation than thiourea R. G. Schulze (1983) German Patent 3347165]. However, the cost of ethylene thiourea is too high to be considered as an economic alternative.